The regulating member of a mechanical watch is formed by a harmonic oscillator, the sprung-balance, whose natural oscillation frequency mainly depends on the inertia of the balance wheel and on the elastic rigidity of the balance spring.
The oscillations of the sprung-balance, otherwise damped, are maintained by the impulses provided by an escapement generally formed by one or two pivoting components. In the case of the Swiss lever escapement, these pivoting components are the pallet lever and the escape wheel. The rate of the watch is determined by the frequency of the sprung-balance and by the disturbance caused by the impulse from the escapement, which generally slows down the natural oscillation of the sprung-balance and thus causes a losing rate.
The rate of the watch is thus disturbed by any phenomena that can impair the natural frequency of the sprung-balance and/or the time dependence of the impulse supplied by the escapement.
In particular, following temporary exposure of a mechanical watch to a magnetic field, rate defects (related to residual field effects) are generally observed. The origin of these defects is the permanent magnetization of the fixed ferromagnetic components of the movement or of the external watch parts and the permanent or temporary magnetization of the moving magnetic components forming part of the regulating member (sprung-balance) and/or of the escapement.
After exposure to the field, the magnetically charged or magnetically permeable moving components (balance wheel, balance spring, escapement) are subjected to a magnetostatic torque and/or to magnetostatic forces. In principle, these interactions modify the apparent rigidity of the sprung-balance, the dynamics of the moving escapement components and friction. These modifications produce a rate defect which may vary from several tens to several hundreds of seconds per day.
The interaction of the timepiece movement with the external field, during exposure, may also result in stopping the movement. In principle, there is no correlation between stopping under a field and the residual rate defect, because stopping under a field depends on the temporary, sub-field magnetization of the components (and thus on the permeability and saturation field of the components), whereas the residual rate defect depends on residual magnetization (and thus, mainly, on the coercive field of the components) which may be low even in the presence of high magnetic permeability.
Since the introduction of balance springs made of very weakly paramagnetic materials (for example silicon), the balance spring is no longer responsible for rate defects in watches. Any magnetic disturbances still observable for magnetization fields lower than 1.5 Tesla are thus due to the magnetization of the balance staff and to the magnetization of the movable escapement components.
The pallet lever body and the escape wheel can be manufactured in very weakly paramagnetic materials without this affecting their mechanical performance. Conversely, the arbors of the movable components require very good mechanical performance (good tribology, low fatigue) to permit optimum, constant pivoting over time, and it is thus preferable to manufacture them in hardened steel (typically 20AP carbon steel or similar). Such steels are materials that are sensitive to magnetic fields because they have a high saturation field combined with a high coercive field. The balance staff and arbors of the pallet lever and escape wheel are currently the most critical components as regards magnetic disturbances of the watch.
Patent Application D1 WO 2004/008258 A2 in the name of DETAR-PATEK PHILIPPE discloses a rotor-stator system composed of a wheel formed of a permanent magnet pre-magnetized in a fixed diametrical direction, and a solution for maintaining an oscillator. This document discloses an arbor producing an electromagnetic torque on which are mounted a rotor and a second pinion, which are not portions of the arbor but are mounted thereon, this arbor being a standard arbor with no specific magnetic properties.
Patent Application D2 U.S. Pat. No. 3,683,616 A in the name of STEINEMANN (STRAUMANN Institute) describes an escapement mechanism wherein all the parts mounted on the balance staff, and on the pallet lever, the escape wheel, and at least the main portion of the balance staff are manufactured from a very weakly paramagnetic material, having a magnetic permeability μ of less than 1.01. A variant concerns the application of a layer at least on the support points of the balance staff. In particular variants, some of the escapement components are formed exclusively from such a very weakly paramagnetic material. The balance spring may be made of such a very weakly paramagnetic material, or of an anti-ferromagnetic metal having a magnetic permeability μ of less than 1.01. In yet another variant, parts mounted on the balance staff are formed from a material selected from the group formed of Monel metal, silver, nickel, copper, a beryllium alloy and a copper-manganese alloy or a nickel alloy. In yet another variant, the pallet lever and the escape wheel are formed from a material selected from the group formed of silver, nickel, a copper-beryllium alloy and a nickel or manganese-copper alloy.
More particularly, the balance staff includes trunnions, and, with the exception of the bearing spindles, is entirely formed from a material having a magnetic permeability μ of less than 1.01. In another variant, the entire balance staff is formed from a material having a magnetic permeability μ of less than or equal to 1.01. The balance staff may also be formed of a hardened bronze.
Patent Application D3 CH 705 655 A2 in the name of ROLEX describes the minimisation of residual effects, i.e. of the difference in rate experienced by a watch subjected to variations in external magnetic fields. This minimisation is correlated, as a surprising effect, with the geometry of the balance staff. More particularly, this document describes an oscillator including a balance spring made of paramagnetic or diamagnetic material, and an assembled balance including an arbor on which are mounted a balance, a roller, a collet integral with the balance spring, and wherein, either the maximum diameter of the arbor is less than 3.5/2.5/2.0 times the minimum diameter of the arbor on which one of the other elements is mounted, or the maximum diameter of the arbor is less than 1.6/1.3 times the maximum diameter of the arbor on which one of the other elements is mounted. This document discloses an arbor having homogeneous intrinsic magnetic properties, in this case a highly ferromagnetic arbor. However, the roller is not an integral part of the arbor.